The present specification relates to spatial light modulators (SLMs) and their operations.
Over the past fifteen to twenty years, micro-mirror based SLMs have made incremental technical progress and have gained acceptance in the display industry. The SLM devices operate by tilting individual micro-mirror plates around a torsion hinge with an electrostatic torque to deflect incident light in a direction that depends on the orientation of the micro-mirror plates. In digital mode operation, each individual micro-mirror plate acts as a pixel that can be turned “on” or “off” by selectively rotating the individual mirror. The mirrors are mechanically stopped at a specific landing position to ensure the precision of deflection angles. A functional micro-mirror array requires sufficient electrostatic torque to overcome contact sticking forces at the mechanical stops and efficient electrostatic torques to control timing and ensure reliability. A high performance SLM device for display applications may produce high brightness and high contrast ratio video images.
Early SLMs in video applications suffer a disadvantage of low brightness and low contrast ratio of projected images. Conventional SLM designs typically have a low active reflection area fill-ratio of pixels (e.g., ratio between active reflective areas and non-active areas in each pixel). A large inactive area around each pixel in the array of SLMs results in low optical coupling efficiency and low brightness. Scattered light from these inactive areas in the array forms diffraction patterns that adversely impact the contrast of video images. Another factor reducing the contrast ratio of micro-mirror array based SLMs is the diffraction of the scattered light from two straight edges of each mirror in the array that are perpendicular to the incident illumination. In a traditional square shape mirror design, orthogonal incident light is scattered directly by the perpendicular straight leading and trailing edges of each mirror in the array during the operation. The scattered light produces a diffraction pattern and the projection lenses collect much of the diffracted light. The bright diffraction pattern can smear out the high contrast of projected video images.
One type of micro-mirror based SLM device is the Digital Mirror Device (DMD), developed by Texas Instruments. Implementations include a micro-mirror plate suspended by a rigid vertical support post on top of a yoke plate. The yoke plate comprises a pair of torsion hinges and two pairs of horizontal landing tips above addressing electrodes. Electrostatic forces on the yoke plate and mirror plate are controlled by the voltage potentials on the addressing electrodes which cause the bi-directional rotation of both plates. The double plate structure is used to provide an approximately flat mirror surface that covers the underlying circuitry and hinge mechanism, which is one way to achieve an acceptable contrast ratio.
However, the vertical mirror support post that elevates the mirror plate above the hinge yoke plate potentially has negative influences on the contrast ratio of the DMD. A large dimple (caused by the fabrication of the mirror support post) is present at the center of the mirror in some designs. The dimple scatters reflected light and reduces optical efficiency. Double plate structures also tend to scatter incident light, which reduces the contrast ratio. Double plate rotation causes a horizontal displacement of the mirror surface along the surface of the DMD, resulting in a horizontal vibration of a micro-mirror during operation. Additionally, the horizontal movement of the mirrors during rotation requires larger gaps to be designed in between the mirrors in the array, reducing the active reflection area fill-ratio. For example, if the rotation of the mirror in each direction is 12°, every one micron between the mirror and the yoke results in 0.2 microns horizontal displacement in each direction. In other words, more than 0.4 microns spacing between adjacent mirrors is required for every one micron length of mirror support post to accommodate the horizontal displacement. The yoke structure can limit the electrostatic efficiency of the capacitive coupling between the bottom electrodes and the yoke and mirror. In a landing position, the yoke structure requires a high voltage potential between the electrodes and the yoke and mirror to enable rotation or angular crossover transition.
Another type of reflective SLM device includes an upper optically transmissive substrate held above a lower substrate containing addressing circuitry. Two hinge posts from the upper substrate suspend one or more electrostatically deflectable elements. In operation, individual mirrors are selectively deflected and serve to spatially modulate light that is incident to, and then reflected back through, the upper transmissive substrate. Motion stops may be attached to the reflective deflectable elements so that the mirror does not snap to the bottom control substrate. Instead, the motion stops rest against the upper transmissive substrate thus limiting the deflection angle of the reflective deflectable elements.
In such a top hanging mirror design, the mirror hanging posts and mechanical stops are all exposed to the illumination light, which reduces the active reflection area fill-ratio and optical efficiency, and increases light scattering. It is also difficult to control the smoothness of the reflective mirror surfaces, which is often sandwiched between deposited aluminum film and LPCVD silicon nitride layers. Film quality determines the roughness of reflective aluminum surfaces. No post-polishing can be done to correct the mirror roughness.
It would be highly desirable to provide a high contrast SLM device that overcomes the foregoing shortcomings.